Immediate objective of this proposal is to describe the particle of tobacco mosaic virus (TMV) and other helical viruses, including the distantly related TMV strain cucumber green mottle mosaic virus (watermelon strain) and the unrelated tobacco rattle virus (TRV), in molecular detail. This forms part of a broader objective: to study the process of macromolecular assembly; in particular, the assembly of macromolecular structures involving proteins and nucleic acids. The present model of TMV has been refined against data at 2.9A resolution to an R factor of 0.10, using the retrained least squares procedure of Hendrickson and Konnert, adapted for fiber diffraction data. Difference Fourier maps have been used to correct the model and locate water molecules. This refinement will continue, and we will continue to improve the difference Fourier technique, which we have recently adapted to take into consideration the special problems of fiber diffraction. The refined model will be used to further elucidate the mechanism of assembly of TMV. Specimens of TMV at low pH will be set up and diffraction patterns recorded. Electron density maps of TMV at low pH will directly reveal the interactions between carboxyl groups from different subunits. These interactions provide a switch, which is used in the assembly and disassembly of the virus. X-ray fiber diffraction studies of CGMMV-W will continue. We plan to search for heavy-atom derivatives in addition to the recently found lead derivative, and to use these derivatives to begin the process of phase determination and calculation of an electron density map, initially at low resolution. Stocks will be built up of the TMV strain U2, for similar work in the future. We plan to improve the quality of our TRV diffraction patterns, by varying the solution conditions under which specimens for X- ray diffraction experiments are made.